OBJECTIVE:: To evaluate S1 transsacral (TS) corridors on reformatted and non-reformatted computed tomography (CT) images to determine which most reliably identifies narrow corridors. DESIGN:: Retrospective cohort SETTING:: Level 1 trauma center PATIENTS:: 245 patients with operative pelvic ring injuries INTERVENTION:: CT scan review MAIN OUTCOME MEASUREMENTS:: Preoperative CT scans were evaluated to determine the width of the S1 TS corridor on standard axial non-reformatted (ANR), axial reformatted (AR), and coronal reformatted (CR) images. Sensitivity and specificity of each format to detect a narrow corridor (<10 mm on AR or CR) was calculated. Patients with S1 TS screws were evaluated to determine the rate of screw breach with narrow corridors. RESULTS:: The axial width of the S1 TS corridor was consistently smaller on ANR vs. AR images (mean difference 1.4, 95% CI 1.1 to 1.5). The corridor width on ANR images was on average 86% of the AR measurement. ANR images had the highest sensitivity and specificity (100% and 98%) for detecting S1 TS corridors <10 mm. 53 S1 TS screws were placed in corridors ranging 10 to 23 mm on AR images and 7 to 19 mm on ANR images. Four (57%) of the seven screws placed in corridors less than 10 mm in width on ANR images breached sacral cortex. CONCLUSION:: Using ANR images to measure the S1 TS corridor consistently measured smaller widths than AR images and identified all narrow corridors. A high rate of screw breach was noted with screw placement in narrow corridors. LEVEL OF EVIDENCE:: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.